Early detection of cancer is generally recognized as the most effective strategy to reduce mortality. However, frequently cancer patients are diagnosed with advanced stage disease which adversely affects clinical outcome. Tumor-derived, circulating proteins can be potentially used as biomarkers for detection of cancer, for monitoring of disease progression, regression and recurrence, and for assessment of therapeutic response. If a test could be developed to detect cancer in its early stages, the lives of many patients might be extended. However, blood has numerous plasma proteins and identification of low abundance tumor-derived proteins is challenging.